Which Lime Is Best?
By: Herb Nordmeyer, author of The Stucco Book—The Basics
We have all heard that mixing lime with Portland cement makes an excellent mortar or stucco, but which lime is best? In answering that question, we have identified the different products that are sold as lime and explained why they are appropriate or not appropriate.
Agricultural lime is calcium carbonate. That is ground limestone and for our purposes is of no value.
Quick lime is limestone that has been heated to about 1,6500 F to drive off the carbon dioxide. It is calcium oxide. When it comes into contact with water, it reacts to form calcium hydroxide. In the process, heat is given off, often enough to boil the water; and if the chucks of quick lime are fist-sized, they may explode. Quick lime can be used to make lime putty, but should never be considered as a component to be added to a mortar mixer.
Lime putty is quick lime that has been hydrated and has a toothpaste consistency. It can be made directly from quick lime, or it can be made by adding water to hydrated lime. It has a place in historic restoration and a few other places, but it is not worth the effort for most mortar and stucco work.
High-cal lime is a hydrated lime that is produced for water purification, wastewater treatment, and many other industrial processes. There are people, including some who should know better, who use it for making mortars and stuccos, but the quality testing is such that sometimes it has oversized (problems can be caused by particles that are less than 1/8″ in diameter) particles that can lead to lime-pops months after a job is complete. The oversized particles are calcium oxide particles, and it takes them a while to hydrate. When they do, the resulting calcium hydroxide takes up more space, so a bit of the mortar is broken off. Usually this results in a conical hole in the plaster or mortar with a white dot in the center. An additional problem with high-cal lime is that it is more prone to causing lime burns than Type S dolomitic hydrated lime.
Type N hydrated lime is very similar to the high-cal lime, but there are more quality checks, and the oversized particles that cause lime-pops are not present. With Type N hydrated lime, over 8% of the lime can be unhydrated. The unhydrated portion may be fine particles that fairly easily hydrate, or it can be hard-burned particles that are very difficult to hydrate. Hard-burned particles usually have a glassy layer around them that takes a long time for the water to penetrate and bring about the hydration process. Since there is unhydrated calcium oxide in the Type N hydrated lime, it can cause lime burns on skin.
Type S hydrated lime has less than 8% unhydrated particles. Much of the Type S is produced from dolomitic limestone (calcium-magnesium carbonate). Since magnesium oxide is harder to hydrate than calcium oxide, the hydration usually is done in a pressure hydrator. As a result, virtually all of the calcium oxide is hydrated, and the magnesium oxide which is not hydrated is less likely to cause skin burns than calcium oxide. Type S hydrated lime particles are usually larger than the Type N hydrated lime particles and give the resulting mortar or stucco more body and more workability. Where it is available, the Type S dolomitic hydrated lime is well worth the extra money it costs.
Herb Nordmeyer, author of The Stucco Book—The Basics